A major U.S. intelligence review has concluded that Iran stopped work on a suspected nuclear weapons program more than four years ago, a stark reversal of previous intelligence assessments that Iran was actively moving toward a bomb.

The new findings, drawn from a consensus National Intelligence Estimate, reflected a surprising shift in the midst of the Bush administration's continuing political and diplomatic campaign to depict Tehran's nuclear development as a grave threat. The report was drafted after an extended internal debate over the reliability of communications intercepts of Iranian conversations this past summer that suggested the program had been suspended.

"Tehran's decision to halt its nuclear weapons program suggests it is less determined to develop nuclear weapons than we have been judging since 2005," a declassified summary of the new National Intelligence Estimate stated. Two years ago, the intelligence community said in contrast it had "high confidence that Iran currently is determined to have nuclear weapons."

The new estimate, prepared by the nation's 16 intelligence agencies, applied the same "high confidence" label to a judgment that suspected Iranian military efforts to build a nuclear weapon were suspended in 2003 and said with "moderate confidence" that it had remained inactive since then.

Even if Iran were to restart its program now, the country probably could not produce enough highly enriched uranium for a single weapon before the middle of the next decade, the assessment stated. It also expressed doubt about whether Iran "currently intends to develop nuclear weapons."

Iran put a stop to weapons-related activities, including efforts to study warhead design and delivery systems, shortly after U.N. inspectors began probing allegations of a clandestine nuclear program. The timing of that decision, according to the intelligence estimate, "indicates Tehran's decisions are guided by a cost-benefit approach rather than a rush to a weapon irrespective of the political, economic, and military costs."

National security adviser Stephen J. Hadley told reporters yesterday that the new conclusions validate the administration's long efforts to pressure Iran, most recently through economic sanctions. Democrats countered that the findings rebutted Bush's portrayal of Iran as an imminent nuclear threat.

Senior officials said the latest conclusions grew out of a stream of information, beginning with a set of Iranian drawings obtained in 2004 and ending with the intercepted calls between Iranian military commanders, that steadily chipped away at the earlier assessment.

In one intercept, a senior Iranian military official was specifically overheard complaining that the nuclear program had been shuttered years earlier, according to a source familiar with the intelligence. The intercept was one of more than 1,000 pieces of information cited in footnotes to the 150-page classified version of the document, an official said.

Several of those involved in preparing the new assessment said that when intelligence officials began briefing senior members of the Bush administration on the intercepts, beginning in July, the policymakers expressed skepticism. Several of the president's top advisers suggested the intercepts were part of a clever Iranian deception campaign, the officials said.

Intelligence officers then spent months examining whether the new information was part of a well-orchestrated ruse. Their effort included "Red Team" exercises in which groups of intelligence officers tried to punch holes in the new evidence, substantially delaying publication of the NIE.

The estimate noted that Iran continues to enrich uranium for a civil nuclear energy program. But the intelligence experts said they did not consider this a weapons program because it is being done at openly declared facilities under international supervision.

If Iran were to proceed with a weapons effort, it would not be carried out at known facilities, the officials said, adding that they do not believe Iran is enriching uranium at an undeclared facility.

Congressional leaders of both parties had been pressing the director of national intelligence, Mike McConnell, for the report for months, and some had worried that the delay was the result of the administration's efforts to influence the final result. Those concerns appeared to dissipate yesterday, when the report contradicted not only the administration's views but also the intelligence community's previous assessments -- evidence, to many observers, of the intelligence agencies' new willingness to question assumptions and assert their independence from policymakers.

"The key judgments show that the intelligence community has learned its lessons from the Iraq debacle," said Sen. John D. Rockefeller IV (D-W.Va.), who chairs the Senate intelligence committee. He was referring to long-standing Democratic allegations that intelligence on Iraq was skewed to help promote the administration's desire for war.

In this case, Rockefeller said, "it has issued judgments that break sharply with its own previous assessments, and they reflect a real difference from the views espoused by top administration officials."

While concluding that Iran's weapons program is now halted, the NIE presents a mixed view of Tehran's nuclear ambitions. It portrays Iran's ruling clerics as susceptible to international pressure, having abandoned an extensive and costly covert nuclear program in the face of threatened economic sanctions and global censure.

But the report also depicts Iran as cleverly preserving its options, by making steady strides toward a civilian nuclear energy capability that both complies with international law and puts the country on a course that will allow it to easily develop nuclear arms if it so chooses.

The report also states more confidently than in previous assessments that Iran's military had been actively seeking to build a bomb. Iranian armed forces were "working under government direction to develop nuclear weapons" until the fall of 2003, it says.

The assessment, under preparation for more than 18 months, was completed on Tuesday and President Bush and Vice President Cheney were briefed on Wednesday, intelligence officials said. Hadley said Bush first learned in August or September about intelligence indicating Iran had halted its weapons program and was advised it would take time to evaluate.

Several participants said there was strong debate among analysts during the process, but in the end they agreed on nearly every judgment. A majority of the intelligence agencies assessed, with high confidence, that the closure of the military program marks the end of the weapons effort. The Energy Department and the National Intelligence Council said gaps in what they know make them conclude only with "moderate confidence" that efforts remain on hold.

The State Department's bureau of intelligence and research judged Iran to be slightly further away from producing enough weapons-grade uranium for a bomb.

Last year, Congress required that key judgments from the NIE be declassified. McConnell said in November that he had no plans to issue an unclassified version, but officials said the dramatic shift in the assessment convinced him otherwise. "Since our understanding of Iran's nuclear capabilities has changed, we felt it was important to release this information to ensure that an accurate presentation is available," Donald Kerr, principal deputy director of national intelligence, said in a statement.